As others have states, this is just a proposal in the Senate right now and has not been passed, so we don't know if it will come to fruition or it could change.
As it's written though:
For married couples filing joint income tax returns, $150,000 is the cap to receive the full $2400 as a couple. If your combined income on your 2018 taxes was between $150,001- $198,000, then you receive a lesser amount based on your earnings over $150,000, with those making greater than $198,000 receiving zero. You also would get $500 per child.
For single adults or those filing separately, the max is $75,000 to receive the full $1200 per adult. Income from $75,001- $99,000 receive a reduced amount, with those earning more than $98,000 receiving zero. Plus you would get $500 per child.
There's also a requirement that you earned at least $2500 in income for the year (social security and veteran benefits count toward that). And individuals or families that had zero or small net income tax liability for 2018 would also get less than the full amount, but at least $600/ individual or $1200/ married, along with the $500 per child.