Used to indicate that something will happen in the near future. Often carries a sense of imminence or expectation.
Spring will be here soon, won't it?
Winter will be here soon.
The train will arrive shortly.
もうじき試験が終わる。
The exam will be over soon.
A guest will be here soon.
"How much longer to the station?" "It's coming up soon."
やがて has a slightly literary, narrative feel and can imply a longer stretch of time than もうじき.
そろそろ often signals it's time to start doing something now, while もうじき points to an event arriving on its own.
Derived from もう (already) + じき (directly, soon). The kanji form 直 is rarely used.