This question asks how to obtain peridicities from a signal. It seems on-topic if very specific to me, dealing with one of the main activities of astronomy (signal processing). However, it was voted down.
I would like to understand why.
Drive-by DVs are frustrating but they are an inevitable part of the Stack Exchange system. Downvotes exist as a means for the SE community to "grade" a question's value. If you hover over the DV arrow, it says "This question does not show any research effort; it is unclear or not useful". There is no obligation for a user to explain why they think the question merits a DV, but we certainly encourage users to add a comment to indicate how the post can be improved. The object should always be to improve a question.
Regarding this particular question, there is (at time of posting this answer) a single downvote. If you want to understand the exact reason, you'll need to hear from the person who cast the DV, but I can hazard a guess.
The timeline shows that the DV was cast early, well before you had asked "Could you please define HTRU in the question, or provide a link to such definition?" Your very comment suggests that the question was lacking sufficient detail to enable a good answer, and was therefore "unclear". [If they had sufficient rep, the user who cast the DV could equally have cast a vote to close the question as "unclear".] In any case, while casting a downvote without explanation is never ideal, the DV itself wasn't without reasonable cause.
The question has now, thanks to your edit, been improved. If you feel the question now "shows research effort; it is useful and clear" (quoted from the guiding text for an upvote), you can cancel out the DV by casting an upvote yourself.
In fact, keep in mind that the person asking the question receives 10 rep points for an upvote but loses only 2 rep points for a DV: the SE voting system primarily treats DVs as an indicator (for improvement needed) rather than as a punishment. And now that the question has been edited, the user who cast the DV can, if they wish, cancel that DV. But again, I wouldn't hold my breath: most DVs never get reversed.
The best way to overcome DVs is to ask good, well-structured questions that attract upvotes, and just accept the occasional unwarranted DV as part of the SE ecosystem, like a briefly irritating flea bite.