When your automatic transmission is low on fluid you'll notice what is called transmission slipping.
When your transmission is slipping it means that the gears in your transmission are not changing as smoothly and as fast as they should.
Your RPM's will go up but the transmission will struggle to change gears. If you notice that problem then make sure to check your transmission fluid level and add transmission fluid if it's low.
If you don't add transmission fluid and continue to drive your vehicle with your transmission slipping then transmission failure is soon to come and that is very costly.
It can cost upwards of $2,000.00 or more to change out a transmission.