Such a box is quite easy with the borderline
options, which draw vertical bars (or other stuff)
Since the note
environment should be versatile, there might be an optional argument that sets other options if needed, see the second (ugly!) example of the note
environment.
The vertical etc. spacings before/after and inside of the box might be change using left=...
etc. and before skip={}
etc. keys.
\documentclass[11pt]{article}
\usepackage[most]{tcolorbox}
\newtcolorbox{note}[1][]{%
enhanced jigsaw, % better frame drawing
borderline west={2pt}{0pt}{red}, % straight vertical line at the left edge
sharp corners, % No rounded corners
boxrule=0pt, % no real frame,
fonttitle={\large\bfseries},
coltitle={black}, % Black colour for title
title={Note:\ }, % Fixed title
attach title to upper, % Move the title into the box
#1
}
\usepackage{blindtext}
\begin{document}
\blindtext
\begin{note}
Here comes a useful note for demonstration. Let's add enough text to break the line, just to be sure the vertical line extends with the text.
\end{note}
\begin{note}[borderline west={2pt}{0pt}{blue}, colback=yellow, drop shadow={red,opacity=0.6}]
Here comes a useful note for demonstration. Let's add enough text to break the line, just to be sure the vertical line extends with the text.
\end{note}
\blindtext
\end{document}

For more sophisticated usages of borderline
etc. please have a look on Reproduce a PDF table with colored vertical semi lines
Here's a version in which the first line of the note
box is aligned with the label of an enumerate environment:
\documentclass[11pt]{scrartcl}
\usepackage{enumitem}
\usepackage[most]{tcolorbox}
\makeatletter
\newtcolorbox{note}[1][]{%
enhanced jigsaw, % better frame drawing
borderline west={1pt}{0pt}{black}, % straigh vertical line at the left edge
sharp corners, % No rounded corners
boxrule=0pt, % no real frame,
fonttitle={\large\bfseries},
coltitle={black}, % Black colour for title
title={Note:\ }, % Fixed title
attach title to upper, % Move the title into the box,
right=0pt,
top=0pt,
bottom=0pt,
frame hidden,
baseline={\tcb@height-2\kvtcb@boxsep+\baselineskip-2\lineskip}, %%%% Needs more to be done for other setups
#1
}
\makeatother
\usepackage{blindtext}
\begin{document}
\blindtext
\begin{note}
Here comes a useful note for demonstration. Let's add enough text to break the line, just to be sure the vertical line extends with the text.
\end{note}
\begin{enumerate}
\item \begin{note}
Here comes a useful note for demonstration. Let's add enough text to break the line, just to be sure the vertical line extends with the text.
\end{note}
\item \begin{note}
Here comes a useful note for demonstration. Let's add enough text to break the line, just to be sure the vertical line extends with the text.
\end{note}
\end{enumerate}
\blindtext
\end{document}

tcolorbox
manual is overwhelming