From Aegisub Manual
The translation assistant is a valuable tool for translating subtitles from one language to the other. It allows you to translate in the most efficient possible manner, by letting you type the translated text without having to worry about selecting the original, or forgetting what was written in it. It also parses ASS override tags, and will skip over them.
The assistant window is simple: it shows you the original raw line, and a box where you can type the translation. There is also a list of hotkeys specific to this window, and a checkbox to enable previewing, if you have the corresponding video set up. Having this option enabled will automatically jump to the current line when you change it.
How to use
Using the assistant is very easy. First, start it by clicking its icon in the toolbar or going to Subtitles->Translation Assistant. It will start at the currently selected line. The original line will have the text that you have to translate highlighted in blue. Simply type the translation to it, and hit enter. If there are override tags in the line, you may have to translate more than one block per line.
In the above screenshot, supposing that you are translating to Portuguese, you would first type "Clima" (Portuguese for Weather) and hit enter. It would then highlight "_______". Since I don't want to translate that, I just hit Insert in the keyboard, and it will copy the text over. So I hit enter, and it will ask for the last bit, "Temperature", so I enter "Temperatura" and hit enter again. Then it will jump to the next line, where I will repeat the process. If I make a mistake, I can use Page Up and Page Down keys to move to the next or previous block/line.