Arbitrage means simultaneously buying and selling related instruments — like spot and Futures of the same underlying — to profit from a temporary, unjustified price mismatch. Arbitrageurs’ trading activity itself helps correct such mismatches quickly, which is part of why Futures and spot prices converge reliably by expiry.
Related Reading
See how arbitrage relates to Futures-spot convergence in Futures Pricing Explained: Premium, Discount, and Cost of Carry.