The LaTeX command for Dashv (⫤) is \Dashv (double turnstile reverse).
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Write \Dashv inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. This symbol needs the fdsymbol package, so add \usepackage{fdsymbol} to your preamble.
\Dashv keeps its spacing inside a formula; for a word such as "and" in the middle of one, use \text{ and } from amsmath. With unicode-math under XeLaTeX or LuaLaTeX, typing ⫤ in a formula gives the same symbol.
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Use \Dashv in math mode after adding \usepackage{fdsymbol} to your preamble. For example, $p \Dashv q$.
It comes from the fdsymbol package: add \usepackage{fdsymbol} to your preamble.