Translation:He is not a girl, she is not a boy.
Romanian is 85% Romance and 15% Slavic, and the only Romance language with 3 genders and 3 cases, similar to Slavic. Also some vowel sounds are similar to Slavic vowel sounds, like the a in barbat.
Short answer: "El" is he, "ea" is she. Este/e is 3rd person singular present tense - he or she is. Ești (it is ș not s) is 2nd person singular present tense - you are. I am not sure where you saw "esta" (as far as I know that isn't part of the verb a fi (to be) in any tense).
The full present tense for a fi (to be) is:
eu sunt - 1st Singular (Sg) - I am
tu eşti - 2nd Sg - You are
el/ea este (or e) - 3rd Sg - he/she is (also use for it is)
noi suntem - 1st Plural (Pl) - we are
voi sunteţi - 2nd Pl - you (plural) are
ei/ele sunt - 3rd Pl - they are (ei is used for groups of all male or mix genders, while ele is used for all female groups)
"Sînt" is older and depreciated form of "sunt" in both 1st sg and 3rd pl, and although it isn't used on Duolingo, you might see it elsewhere, especially in older sources or older people using it. For example, Sebastian Stan, the actor, still uses the old form because he left Romania just before the spelling was changed to "sunt". I believe though that it's still pronounced the same as sunt.
"E" is just a contraction of "Este" and they are totally interchangeable, although "e" is a little less formal - very similar to "it's" versus "it is".
With many verbs you don't need to use the pronoun (i.e. eu, tu etc) as you can tell what "person" the verb is based on its spelling. For example, saying "ești" would automatically tell you that it it 2nd person singular, "you are".
Similarly, sometimes you can tell from context. With 3rd person singular "este" you might think you always need the pronoun, but sometimes context makes it clear without it. For example "Este bun" or "Este un băiat" would automatically tell you that in that context, "este" means "he is" because bun is the masculine form of "good" and un băiat is "a boy". Similarly "Este bună" or "Este o fată" would tell you that means she is, because they are the feminine form of "good" and "a girl" respectively. If you wanted to say she is a boy, or you want to be clear, then you would use "el" or "ea" to clarify. "Este" can also mean "it is" too - so "Este mare" could mean any of "he/she/it is big".
The same is true for telling whether sunt, without a pronoun, means "I am" or "they are". Sometimes you just need to use a pronoun but often, the rest of the sentence or wider context would make it clear. "Sunt o fată" would make it clear that it means "I am" because "o fată" means "a girl", which is singular. Similarly "Sunt fete" would tell you that it means "they are, because "fete" means "girls", plural. Again, if you wanted to say, for example, "I am many girls", you would probably have to use the personal pronoun to be absolutely clear, though again, wider context might make it implicitly clear, if you were having a longer discussion about yourself, for example, rather than a single standalone phrase/sentence. The same works for adjectives, as with 3rd singular - "Sunt bun" or "Sunt bună" would be "I am good" because those are the masculine singular and feminine singular forms of "good" respectively, and "Sunt buni" or "Sunt bune" would be "They are good" because those are the masculine or mixed group and feminine plural forms of "good" respectively.
Sorry if that is way more information that you needed!