SHA1 is a one-way cryptographic hash function which has several purposes in computer programs. Released in 1995 by the NSA, it converts an input string of any length to a 160 bit output value, which is usually represented as a 40 character hexadecimal number such as aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d, which happens to be the "SHA1 hash" of the word "hello".
In modern times, SHA1 is still "pretty secure" but it isn't recommended for use cases which require the utmost security. But it is fine to use in cases such as data validation. In fact, the Git version control system uses SHA1 to identify versions of a code base. For more information, see this wikipedia article on SHA1.