Glean means to gather bit by bit, either literally or figuratively. You might glean leftover grain from a recently harvested field or glean information about new vocabulary words by hearing them used in context.
When you glean information, you sort through ideas and take what you need. Seeing a word in context lets you glean information about how it's used, for example. If you want to find treasures at a thrift shop, you’ll have to glean the good stuff out of the piles of junk. To glean a field means to walk along and see what’s left on the ground. It takes patience to glean.