What many on the left assume it means, is a sincere description of growth "raising the tide"; and how ridiculous an idea that is.
Of course the tide has risen to an unprecedented degree in human history both since the term "trickle down" was invented; and moreso, over the last 40 years.
The term was coined by Will Rogers about 90 years ago.
This is a side effect of using the same category terms, but not talking about the underlying distributions.
Eg., in the UK, 80% of the country were industrial working class or poorer until 1980s; and middle class only started at top 5%.
Today, the "industrial working class" level of wealth, is the bottom 20% at the very most.
So if you hold the class terms fixed, "middle class" it seems the 80s top 5% has "stagnated", only to fail to mention, it now 60% of the country.
If you hold the distributions constant, the wealth level acheived in the top 5% in 1980s is now a majority of the population.
"You" in the above is a specific individual.
"They" in the above is undefined and extremely imprecise.
For various definitions of "they", your statement could be true, false or somewhere in between.
Trickle-down economics is not explicitly endorsed by anyone, because it's a pejorative. That's like saying "totalitarianism is popular in some countries"; you might think those countries are totalitarian, but they don't.