Sold STC means sold subject to contract.
In other words, the seller has accepted the buyer's offer price, but that the sale contract has not been signed by the parties.
At this stage, there is still the possibility that the sale might not complete, so if you are interested in the property, you should still contact the agent to express your interest. If the sale does not go through, you might be in line as the next possible buyer.
Why advertise a property as Sold STC?
Estate agents like to show properties as Sold STC for marketing purposes. There is no legal basis for the practice.
By showing a mixture of properties without offers and with offers in their windows (on on their websites), agents effectively advertise:
that they handle a wide range of properties, including ones like a prospective seller might sell and ones that a prospective seller might buy
that they are successful in finding buyers (which for sellers is a reason to use their services, and for buyers a reason not to delay in registering their interest)
that they are so successful that they don't have time to take down properties that they have sold
that they act as agent for more properties than they actually do
If you are interested in a property advertised as Sold STC
Whether or not you can view the property will be up to the seller.
In theory, he or she has already agreed to sell to someone else, so may not want to show you around (either because it takes his or her time and he or she does not feel your viewing is worth the time, or because he or she feels that it is unethical to entertain the possibility of selling to someone else).
However, some sellers may allow a viewing if:
they think you might make a higher offer
you are in a more attractive buying position (for example, you are not in a chain and already have a mortgage agreed in principle)
the current buyer is taking too long
You should talk to the agent about your interest. There may be a chance you can view the property then make a more attractive offer (known as gazumping).
Even if the seller prefers to continue with the current sale, the agent may be able to find other similar properties that suit your requirements.