Meaning:
“网上” means “on the Internet,” and “谈兵” means “talking about war strategies.” This term was based on a long existing idiom “纸上谈兵” (literally, “talking about war strategies on the paper”). That idiom refers to Zhao Kuo, a general of the Zhao Kingdom during the Warring States period (475-221 B.C.) who lost a watershed battle to Qin armies at Changping because he strictly followed the strategies taught in books instead of making adjustments in reality. The new idiom jokingly coined by netizens refers to those people who think it’s easy to learn stuff by watching videos online but do it poorly in real life.
Example:
A: 小李最近忙什么呢？
Xiǎolǐ zuìjìn máng shénme ne？
What’s Xiao Li up to lately?
B: 他买了游泳的各种装备，准备学游泳。
Tā mǎile yóuyǒng de gèzhǒng zhuāngbèi，zhǔnbèi xué yóuyǒng。
He’s bought all kinds of swimming gadgets and is planning to learn swimming.
A: 我看他又是网上谈兵，上次学网球不就上网看了不少视频，结果一看就会，一学就废。
Wǒ kàn tā yòushì wǎngshàng tánbīng，shàngcì xué wǎngqiú bú jiù shàngwǎng kànle bùshǎo shìpín ，jiéguǒ yīkàn jiùhuì，yīxué jiùfèi。
I guess he’s learning it online again. Last time he tried to learn tennis and watched a lot of videos online, but he cannot play at all on court.