The default usage terms are "all rights reserved, nobody can use this but me". You change this by applying licenses which regulate the terms under which you're

The default usage terms are "all rights reserved, nobody can use this but me". You change this by applying licenses which regulate the terms under which you're happy for the code to be used by others. The vast majority of popular Free Software licenses allow you to use the code under no guarantees whatsoever, so if you want to use some software in critical applications and hold its authors responsible if it doesn't work as advertised, you should probably pay them and include this responsibility in their contract.