Frequency is a term used to describe waves in nature. There are waves in an ocean. Then there are sound waves and then there are light waves. All these waves rise and fall. Waves come and go. They don't stand still. You can see the picture below.

How quickly these waves come one after the other is called frequency. The more waves you see in a given amount of time, say in 1 minute or in 1 hour, then "more" is their frequency. So a frequency is the number of waves you see in 1 sec.

If you have a stop watch and you are standing in the sea you can count the number of waves that hit you in 1 sec. Then that number is the frequency of waves at that time of the day.