A hash is like a unique fingerprint for data or for a file. MD5, SHA1, and SHA256 are different ways to create unique fingerprints. SHA256 is the strongest and most secure one.
A file hash can be used to check whether a program file has been modified or corrupted.
Examples from Notepad.exe:
•MD5: E92D3A824A0578A50D2DD81B5060145F
•SHA1: 50EF7C645FD5CBB95D50FBADDF6213800F9296EC
•SHA256: 87F53BC444C05230CE439DBB127C03F2E374067D6FB08E91C834371FD9ECF661
