What it can check, and what it can't
It can check: whether the prefix is right (like 0x on Ethereum-style chains or T on Tron), whether the characters belong to the allowed set, and whether the length falls in a sensible range. That catches most of the obvious slips — a mistyped character, or a few characters lost while pasting.
It can't check: it doesn't verify that the checksum is mathematically correct, it doesn't look up whether the address really exists on-chain or holds a balance, and it can't tell whether the other side's network supports the coin you're sending. That part is on you: double-check by hand, then send a small test. To understand what happens when an address is wrong, read Can you recover a withdrawal sent to the wrong address? If you're worried an address got swapped, read Spotting clipboard hijacking.
Malware swaps your address for a different one that's still perfectly valid — this tool can't catch that, because the format is fine, it's just not the address you meant. So "format passes" is not the same as "address is correct." After pasting, always compare it character by character against the original the other side gave you.
Once you're sure of the address, use Pick a network to choose the chain, and the memo checker to see whether you need a tag.