Most Constructs have the ability text saying "May only Summon onto Land."
During the Summon Phase, if you can somehow Summon a Land into the Tableau, you may then Summon the Construct onto the Land. Usually, the combined Land and Construct will then be moved to your Realm. This is a great way to layaway Constructs that normal clog up your Stockpile.
In most games, you will not be able to Summon a Land during your Summon Phase. However, there are ways that this can be done.
One example is by playing a Hippocamp.
When you Summon the Hippocamp, it allows you to then Summon a Land from your Tray at no Resource cost. Once the Land has been Summoned and is in the Tableau, you may then Summon a Construct onto it (if you have one in your Tray).
Another example is via the Grounds Environment.
The Grounds Environment gives any Lands with a Resource Value of 2 or more, a Mana Cost such that it can be Summoned during the Summon Phase. This means you can play your Lands like you do your Creatures, and once you've Summoned a Land, you can then Summon a Construct onto it.
You can check your Realm and see the Land/Construct combos you've done.