A strike price is the set price at which an option contract can be bought (call option) or sold (put option) when it is exercised. For call options, the strike price is where the asset can be bought by the option holder; for put options, the strike price is the price at which the asset can be sold.
The strike price is a key variable of call and put options. For example, the buyer of an ETH call option would have the right, but not the obligation, to buy that ETH in the future at the strike price. Similarly, the buyer of an ETH put option would have the right, but not the obligation, to sell that ETH in the future at the strike price.