Less than, when used in a mathematical context.
Example: x < y; this is read as x is less than y

Left angle bracket in other contexts, in html and other markup languages, this is used to open a directive.

In Bash (and i'd imagine other shells as well..), < is used to redirect the input of a program away from the keyboard to a file. For example typing command < filename causes command to run, with the contents of filename being fed to command as if the user had typed them in.