Means that the transmission is delaying going into the next gear.
Transmission temp sensor can cause a slip or high rpm before shifting to next gear.
because the transmission is going out, it need rebuilt or replaced
Yes. From low fluid, plugged filter, or an internal failure.
A 1994 Mazda Miata's automatic transmission may slip in 4th gear if the bands are heavily worn. When the bands become worn, they are unable to grip properly and cause slippage.
AT can mean Automatic Transmission.
I'll assume you mean an automatic. If so, there are friction plates inside the transmission that get worn out and just like old sand paper gets smooth and doesn't do it's job very well, the friction plates get smooth and don't grip anymore, causing the transmission to slip. In the possible case it is a standard transmission, it isn't the transmission, it's the clutch slipping.
The bands may be weak, it may be low on fluid.
automatic Transmission Fluid
automatic transmission fluid
Automatic Transmission
Most likely that it comes with an automatic transmission opposed to coming with a manual transmission and then having to pay extra for the automatic.
If the gears are slipping on a 1995 Honda Civic then the transmission is worn. The transmission will either soon need to be replaced or repaired.