You should try some or all of the following:
Speak to the chair before the meeting starts and explain that you are attending as a deputy and haven't been briefed - and ask for any background information the chair thinks may be helpful to you
During any introductions, explain the situation to everyone in the meeting (unless the chair does that for you)
If any questions arise about the progress made by the person you're deputising for, respond by saying that you'll have to revert back once you've spoken to them - don't bluff or guess any answers
If any actions are suggested for you, remind the meeting that you're a deputy and hadn't expected to personally pick up any actions
If any actions are suggested for the person you're deputising for, explain to the meeting that you can't commit them to any deadlines without speaking to them first
Take copious notes and make sure you brief the person you deputised for as soon as possible after the meeting
If appropriate, arrange to attend the next meeting along with the person you deputised for, to ensure continuity