Hey there! I'm happy to help!
The Treaty of Tordesillas was a treaty between Spain and Portugal that put a line of demarcation at a certain line of longitude that cut through Brazil to show which country could claim which territory. Spain could claim territories to the West and Portugal could claim territories to the East of this line.
This affected exploration because it limited Portugal's ability to colonize the Americas while it led Spain take the jackpot of the Americas. However, Spain couldn't colonize East of the line of demarcation, so it was a fair compromise.
This was made after Columbus explored the Americas, and Spain didn't want Portugal interfering, so they claimed all the Americas for themselves but limited themselves from exploring areas East of the demarcation line.
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