How do the words mean, median and mode mean?

The mean is the same as averaging. If you don't understand the concept of how to average, allow me to explain.
The average (mean) is when you take the sum of all numbers in a given sequence and divide the sum by how many total numbers you have.

For example;
1, 2, 3, 4, 5.
Add the numbers up.
1 + 2 = 3
3 + 3 = 6
6 + 4 = 10
10 + 5 = 15
Our total sum is 15.
We added a total of 5 numbers, so divide our sum by the total numbers.
15 / 5 = 3.
Your mean (average) is 3.

The median is the middle number of a given sequence.

For example;
1, 2, 3, 4, 5.
The most efficient way (in my opinion and experience) is to remove the smallest number and the largest number in pairs until you can no longer do so.
1, 2, 3, 4, 5.
Remove the smallest number, 1, and remove the largest number, 5.
2, 3, 4.
Remove the smallest number, 2, and remove the largest number, 4.
3.
The median for 1, 2, 3, 4, and 5 is 3.

The mode is the number in a given sequence that appears the most frequent.

For example;
1, 2, 3, 3, 4, 5.
Let's look for the number that appears the most in our sequence.
1 only appears once.
2 only appears oncs.
3 appears twice.
4 only appears once.
5 only appears once.
3 appeared the most out of all the numbers, making 3 the mode.

I hope this helps!