The amount of trust you have in the key ownership and the care the owner puts into signing other keys. The available levels are
unknown or 1 : I don't know or won't say.
never or 2 : I do not trust.
marginal or 3 : I trust marginally.
full or 4 : I trust fully.
ultimate or 5 : I trust ultimately.
See the Key Management chapter of the GNU Privacy Handbook for more.