Naming carbon trains
Picture carbon atoms as train cars linked in a line — count the cars and you get the name: one is 'meth-', two 'eth-', three 'prop-'. Organic nomenclature is just a system for reading how long the train is and what's attached.
Chains of carbon
Hydrocarbons contain only carbon and hydrogen. Alkanes have only single C–C bonds; alkenes contain at least one C=C double bond.
Prefixes count carbons
meth- = 1, eth- = 2, prop- = 3, but- = 4. So 'propane' is a 3-carbon alkane.