That would be "Ihr esst Früchte".
"Obst" is a mass noun (uncountable) and corresponds to the mass-noun usage of English "fruit".
So if you are eating two strawberries, a cucumber, and a slice of watermelon, all that together would be "You are eating fruit" = "Ihr esst Obst".
"eine Frucht" is one fruit. It's a single, distinct object.
You can also have many "Früchte", meaning you have multiple single fruits.
For example, two strawberries and a banana are "drei Früchte".
"Obst", on the other hand, is more of a general idea of fruit. Like (for example) "food" in English, it's not countable -- it can refer to part of one fruit, to parts of many fruits, to several complete fruits.
So you can use it when the quantity is not important.
For example, "Es ist gesund, jeden Tag Obst zu essen." (It's healthy to eat fruit every day.) It doesn't matter how many individual fruits you eat -- it's the point of eating fruit in general that's healthy.
So you could say that it's like the general meaning of "fruit".
But if you want to say how many (whole) fruits there are, you can use "Frucht, Früchte": "Im Korb waren sieben Früchte" means that there were seven (whole) fruits in the basket.
"ihr" = many people
"du" = one person
For example, "du bist mein Freund" (you - one person - are my friend) versus "ihr seid meine Freunde" (you - several people - are my friends).
In both case, it's an informal way of address -- roughly, for people with whom you are on first-name terms.
For people that you are more formal with, the address is "Sie" (whether for one person or for several): "Sie sind mein Freund" / "Sie sind meine Freunde".
Because the verb is essen with an e in the stem.
The du and er, sie, es forms sometimes change in the vowel in some verbs (including this one) but the other forms (including ihr) always use the basic stem.
So for example, sehen, essen, brechen, werden, sprechen, geben, lesen, fahren, wachsen, laufen have du siehst, er isst, du brichst, er wird, du sprichst, er gibt, du liest, er fährt, du wächst, er läuft with vowel change, but ich sehe, wir essen, ihr brecht, sie werden, ich spreche, ihr gebt, wir lesen, sie fahren, ich wachse, ihr lauft with the basic vowel.
(Another exception is some preterite-present verbs, particularly modal verbs, where the ich form can also change, e.g. können, mögen, wissen which have ich kann, mag, weiß - but here, ihr again uses the basic stem: ihr könnt, mögt, wisst.)
In the first sentence, the speaker eats the fruit and in the second sentence, the listener eats the fruit :)
- I eat fruit: Ich esse Obst
- you eat fruit: du isst Obst / ihr esst Obst / Sie essen Obst (depening on the number of people you are talking to and how well you know them)