I copy and paste your expression into a notebook and it is like this,

Since this sum will evaluate, I wrap it in HoldForm
and then apply TeXForm
,

I copy this expression and wrap it in $$
, like this
$$\sum _{\eta =1}^{\tau } \frac{\lambda ^{\eta }}{\eta }$$
and on the main site, but not meta apparently, and it renders as

This lets me put a math expression in a stack exchange post. But if I wanted to post usable code that is also readable, I could replace TeXForm
with InputForm:

which, when copied and pasted here gives
HoldForm[Sum[\[Lambda]^\[Eta]/\[Eta], {\[Eta], 1, \[Tau]}]]
This is already an improvement, but if you want to go a step further in readability, you can use the plugin described here (as pointed out by Rohit in comments), and get
HoldForm[Sum[λ^η/η, {η, 1, τ}]]