There are two main ways to make use of the L1 and L2 GPS signals. First, the L1 signal can be used on its own. This will give you about a centimeter of accuracy (when also using RTK) but is still considered slow, taking about 10 minutes to reach that level of accuracy.
The L1 and L2 GPS signals can also be used together in a method called PPP (Precise Point Positioning). Since the L2 signal has a higher frequency, it can travel much more easily through obstacles. This means that errors caused by particles in the air can be calculated and eliminated by comparing the two signals. This gives you a level of accuracy comparable to when using the RTK method, except only one receiver is needed. Additionally using the L1 and L2 together allows the receiver to operate in tougher conditions, such as near buildings or under tree cover (source). Since the L2 signal is faster and sent out more frequently, this method also allows for faster initial signal acquisition than with L1 alone.