I would never say "it explains a lot". Only "that explains a lot" You would only use it if an explanation was somewhat unexpected and it explained something that was a puzzle. Why did she get hired? She was the only candidate for the job who spoke English. That explains a lot. (No) Why did she get hired? She doesn't even speak English and doesn't seem to know what she is doing. She's the boss' niece. That explains a lot. (Yes)

It is much more common and natural to say “That explains a lot” or sometimes we say “That explains it then”

'Explain' needs to be plural. It's, "It explains a lot", or "That explains a lot". You are confused or don't understand why someone did or is doing something. You are given the reasons and now you understand, and you say 'It explains a lot'. Example: Background: I don't really understand why a colleague at work is always absent. Someone tells me that this collegue has a serious illness. I reply, "Ah, I understand. That/it explains a lot." (I'm saying, that explains why they are always absent)

The answers you've already gotten are good. I just wanted to clarify that Greg meant to say "singular," not "plural." (The subject, "it," is singular, so you need to say "explains," which is the 3rd person singular form of the verb. You can use the plural form if you have a plural subject: "The things you said explain a lot." Since the word "things" is plural, you use the plural verb form, "explain," in this case.)

Hello Ethan, the correct phrase is : "It explains a lot", this phrase is usually used after something has been explained to you, or after you have had a discussion with someone about something which you did not understand or were not sure about. Hope this Helps. Have a great day.