diff --git a/src/validation.h b/src/validation.h
--- a/src/validation.h
+++ b/src/validation.h
@@ -265,7 +265,7 @@
  * for non-network block sources and whitelisted peers.
  * @param[out]  fNewBlock A boolean which is set to indicate if the block was
  * first received via this call
- * @return True if state.IsValid()
+ * @return true if the block is accepted as a valid block
  */
 bool ProcessNewBlock(const Config &config,
                      const std::shared_ptr<const CBlock> pblock,
@@ -325,7 +325,15 @@
 bool GetTransaction(const Config &config, const uint256 &hash,
                     CTransactionRef &tx, uint256 &hashBlock,
                     bool fAllowSlow = false);
-/** Find the best known block, and make it the tip of the block chain */
+/**
+ * Find the best known block, and make it the active tip of the block chain.
+ * If it fails, the tip is not updated.
+ *
+ * pblock is either nullptr or a pointer to a block that is already loaded
+ * in memory (to avoid loading it from disk again).
+ *
+ * Returns true if a new chain tip was set.
+ */
 bool ActivateBestChain(
     const Config &config, CValidationState &state,
     std::shared_ptr<const CBlock> pblock = std::shared_ptr<const CBlock>());
@@ -539,7 +547,12 @@
 
 /** Functions for validating blocks and updating the block tree */
 
-/** Context-independent validity checks */
+/**
+ * Context-independent validity checks.
+ *
+ * Returns true if the provided block is valid (has valid header,
+ * transactions are valid, block is a valid size, etc.)
+ */
 bool CheckBlock(const Config &Config, const CBlock &block,
                 CValidationState &state, bool fCheckPOW = true,
                 bool fCheckMerkleRoot = true);
@@ -598,7 +611,14 @@
 CBlockIndex *FindForkInGlobalIndex(const CChain &chain,
                                    const CBlockLocator &locator);
 
-/** Mark a block as precious and reorganize. */
+/**
+ * Treats a block as if it were received before others with the same work,
+ * making it the active chain tip if applicable. Successive calls to
+ * PreciousBlock() will override the effects of earlier calls. The effects of
+ * calls to PreciousBlock() are not retained across restarts.
+ *
+ * Returns true if the provided block index successfully became the chain tip.
+ */
 bool PreciousBlock(const Config &config, CValidationState &state,
                    CBlockIndex *pindex);
 
diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -2389,7 +2389,9 @@
     FlushStateToDisk(chainparams, state, FLUSH_STATE_NONE);
 }
 
-/** Update chainActive and related internal data structures. */
+/**
+ * Update chainActive and related internal data structures when adding a new block to the chain tip.
+ */
 static void UpdateTip(const Config &config, CBlockIndex *pindexNew) {
     const Consensus::Params &consensusParams =
         config.GetChainParams().GetConsensus();
@@ -2901,11 +2903,6 @@
     }
 }
 
-/**
- * Make the best chain active, in multiple steps. The result is either failure
- * or an activated best chain. pblock is either nullptr or a pointer to a block
- * that is already loaded (to avoid loading it again from disk).
- */
 bool ActivateBestChain(const Config &config, CValidationState &state,
                        std::shared_ptr<const CBlock> pblock) {
     // Note that while we're often called here from ProcessNewBlock, this is
@@ -3618,6 +3615,11 @@
     return true;
 }
 
+/**
+ * If the provided block header is valid, add it to the block index.
+ *
+ * Returns true if the block is succesfully added to the block index.
+ */
 static bool AcceptBlockHeader(const Config &config, const CBlockHeader &block,
                               CValidationState &state, CBlockIndex **ppindex) {
     AssertLockHeld(cs_main);
@@ -3793,9 +3795,9 @@
                      block.GetHash().ToString());
     }
 
-    // Header is valid/has work, merkle tree and segwit merkle tree are
-    // good...RELAY NOW (but if it does not build on our best tip, let the
-    // SendMessages loop relay it)
+    // Header is valid/has work and the merkle tree is good.
+    // Relay now, but if it does not build on our best tip, let the
+    // SendMessages loop relay it.
     if (!IsInitialBlockDownload() && chainActive.Tip() == pindex->pprev) {
         GetMainSignals().NewPoWValidBlock(pindex, pblock);
     }