Checking every signature in Bitcoin's history is the slowest part of syncing a new node. Assumevalid, added in Bitcoin Core 0.14.0 (2017), is a shortcut.
How it works
Each release hard-codes the hash of a recent block. If that block is in the chain the node is syncing, and the chain has enough proof of work, the node skips script and signature checks for that block and its ancestors.
What is still checked
Everything else: proof of work, block structure, amounts, that every input spends an existing coin, and the full UTXO set is still built. After the assumed block, everything is checked in full.
Trust
The value is reviewed in public like any other code change. Anyone can turn the shortcut off with -assumevalid=0.