Learn Dutch: Ik meen het echt.

English uses "to mean" for several different things, but Dutch splits them up. You use betekenen for the definition of a word or sign. You use bedoelen to explain what you are trying to say or refer to. You use menen when you want to express that you are being completely serious and sincere, which is exactly what is happening in this sentence.

Why is echt at the end of the sentence, while "really" comes before the verb in English?

In a standard Dutch main clause, the conjugated verb must take the second position. Here, Ik is in the first position, so the verb meen has to come second. Adverbs like echt generally get pushed later in the sentence.

Could I say Ik meen echt het instead?

No, you cannot. In Dutch word order, specific object pronouns like het (it), hem (him), or haar (her) almost always come immediately after the verb, jumping ahead of adverbs like echt.

The verb meen has a double "e", but its dictionary form menen only has one. Why does it change?