This phrase is used to exaggerate how good someone is compared to other people.
Imagine that you could compare people's skills and talents mathematically. If there were 10 team members, all with a skill level of 8 points, and one other team member with a skill level of 90 points, then you could say that the last team member is "better than all the other team members put together":
10 x 8 = 80
80 < 90
This is the idea that you communicate when you use this phrase.
People use the phrase "better than ___ put together" pretty often. You can use it to compare someone's abilities, intelligence, wealth, beauty, etc.:
Sometimes it seems like she's smarter than the rest of the entire family put together.
You can also use it to compare objects as well:
The sun is many times larger than all of the planets in the solar system put together.