Open interest is the number of contracts that exist and haven't yet closed. Volume tells you how much traded today; open interest tells you how much is still out there, and where.
Where the positions are
Large open interest at a strike doesn't by itself say which way people are betting: every contract has a buyer and a seller. It does tell you where hedging flows and expiry effects are likely to concentrate. Very large strikes can act as magnets or walls as expiry nears, particularly in quiet markets.
Max pain
Max pain is the expiry price at which the total payout to option buyers would be smallest. The idea is that option sellers, often professional market makers, benefit if price settles there. Treat it as a description of the positions, not a forecast. Price has plenty of other reasons to go elsewhere, and it often does.
Put/call ratios
The put/call ratio compares open interest or volume in puts and calls. It's commonly read as a sentiment gauge, but many puts are hedges against existing holdings and many calls are sold against them, so the ratio alone says less than it seems to.