How to guide on gear ratios:
The chain ring is the name for the ring the chain sits on in the front of the drivetrain or by the pedals. Every chainring has teeth on it, though the number of teeth on a given chain ring changes.
On the back of the drivetrain where the chain sits by the wheel, you have what is called a rear cog or sprocket. These will also have teeth on them.
The more teeth on the chainring, the higher gearing you have. Think of it this way: the more teeth your have on the chainring, the harder it will be to pedal the bike. The less teeth on the chainring, the easier.
On the rear cog or sprocket, the opposite logic applies. The more teeth on the rear cog, the easier it will be to pedal. The less teeth on the rear cog, the harder it will be.
On the bike in this video, the rear cog has 16 teeth and the chainring has 42 teeth.
In this case, 16t:42t will be your gear ratio.