Generally pasting a register is done from Normal mode using p or something like '1p (the former pasting from the default register, 0, and the latter pasting from register 1). Vim also allows you to paste from a register without leaving Insert mode. By hitting CTRL-R and then the name of the register, Vim will insert the contents of the register in front of the cursor.

For example, to paste from the default register from Insert mode, hit CTRL-R 0.

Note, mappings and abbreviations will not be applied to the inserted text.

See :h i_CTRL-R for more details.

h/t Chris Erin