What's a subjunctive?

It's one of the verbal moods available in English, along with the indicative ('I eat this') and imperative ('eat this!'). It is always used in conjunction with the main clause: 'I suggest he eat this'. In English, it's pretty much vestigal in most cases, but it's important to remember its existence when you use words like recommend, suggest, advise etc. with 3rd person singular ('I recommend Dan do the training').
It also exists in the second conditional structure, also only in the 3rd person singular: 'I would prefer it if she were here'. These specific cases are the only two instances you will find the subjunctive in contemporary English; it used to be much more widespread before, but it's now considered archaic and very formal ('I will ensure that he leave immediately').