First of all, don't check that if you're deeding land that is on a private sim (not mainland), it's not needed and could only cause problems.
When you deed land to a group then you technically no longer own the land, the group does. However, groups by themselves have no way of providing/paying for the tier that is needed to keep the land around so you would need to donate (some of) your own personal tier to the group in order to keep up the land.
(In a true group effort, many people could donate small amounts of their personal tier to keep the land up)
What you would normally do is open the group window, go to the Land & L$ tab and put in the required amount of m² of tier as your donation since land can not be deeded to a group without the group being able to support it tier-wise. Tier donated to a group is always considered as "actively used" though, so in order to deed land to a group you'd actually need twice the amount of tier (if that makes sense, you know everything you need to know really

).
"Owner makes contribution deed" simply combines donating tier and deeding the land into one single step.
---
If you were to sell your then group-deeded land, your tier will remain donated to the group. If you wanted to buy a new bit of land, instead of buying directly, you'd go to "World / About Land" (and something that people easily forget, make sure you have the group you want to buy for as your active group) and click "Buy for group".
(If you wanted to get rid of your land entirely and tier down, you need to clear your tier donation in the group)
---
(Oh and donating tier to a group means you end up with 10% extra... so if you donated 1024m² worth of tier the group could actually own 1126m² of which 1120m² is actually usable since land comes in multiples of 16m². In the case of checking "Owner makes contribution", SL just donates enough for the group to support the land, so it would donate 931m² which would support the 1024m²)
Or you could read
https://support.secondlife.com/ics/support/KBAnswer.asp?questionID=4041
and
https://support.secondlife.com/ics/support/KBAnswer.asp?questionID=4062
which should be less confusing than my explanation

.