Refers to the main point or general meaning of a text, speech, or argument, rather than a detailed account.
彼の話の大意は理解できた。
I could grasp the gist of his story.
この文章の大意をまとめてください。
Please summarize the main points of this passage.
概略 gives a brief outline of events or plans, while 大意 refers specifically to the general meaning or gist of a piece of writing.
Compound of 大 (big, general) and 意 (meaning, intention), literally 'broad meaning'.