The [H] float option turns the float (figure, table, etc.) effectively into a non-float (although it may have a regular caption) making it appear in the final document where it was put in the LaTeX source. This can then result in much extraneous white space (what happens if the [H] float requires 3 inches vertical space but there is only 2 inches available on the page?).
If you really want a float to be a non-float then don't call it as a float e.g.,
%%% begin{table}% don't use this!
\begin{tabular}
% tabular code
If you need a caption then you can use the caption package. For example
\usepackage{caption}
% etc
\begin{center} % if you want the tabular centered
\begin{tabular}
% etc
\captionof{table}[LoT entry]{Nonfloating table caption}
\label{tab:X}
\end{tabular}
\end{center}
% etc
LaTeX tries very hard to position floats in a pleasing way via the optional float parameters [htbp]. If you think that you can do better then good luck to you.
A comprehensive discussion about floats and what you can do with them is in the 43 page Chapter 6 of Frank Mittelbach and Michael Goossens The LaTeX Companion, Second Edition, Addison Wesley, 2004 (I understand that a Third Edition may be in the offing at some future point). The chapter includes details of how you can adjust the allowable spaces for top (t), bottom (b) and here (h) floats.
The memoir manual (>texdoc memoir) has a Chapter Floats and captions that contains similar information. The Not so Short Introduction to LaTeX2e (> texdoc lshort) also has useful information on the topic.
Following from jfbu's comment I should have remembered that Frank Mittelbach wrote a comprehensive article How to influence the position of float environments like figure and table in LaTeX? It can be accessed, and printed from, [TUGboat, the journal of the TeX User Group]{https://tug.org/TUGboat/tb35-3/tb111mitt-float.pdf}
[H]isn't mentioned once. This post is a bad fit and isn't phrased as a question but it isn't a duplicate of that one. – David Carlisle Jan 12 '19 at 22:59[h]is heinous, not[H]. I mean, I sometimes do use the[H]specifier, but I always avoid[h], which could block the queue of the type of floats being used. – GuM Jan 12 '19 at 23:40