What the meaning of 线上引流？

线上引流: 通过在线上(网络上) 做一些事情, 引入更多的客流.

线上: this is a adjective, not a noun. by the way of online...

引: 吸引, attract

流: 客流,人流. people, customers.

In English , its meaning is: to get more people/users/customers by doing something online. this word is widely and commonly used by the internet companies.

It means "to attract traffic".

It's a term of online marketing / e-commerce, highlighted also by the terms "线上 = online" and "线下 = offline".

线上引流、线下交易

attract traffic online, convert offline ("convert" as in customer, or lead, conversion)

Your understanding, as also others pointed out, is essentially correct, but I believe the contraction is from：

吸引流量 (流量 = traffic）

which makes more sense, if you look at it from the point of view of typical Chinese chiasm abbreviations:

（吸）引流（量）

omitting the least semantically relevant character of each word, without having to specify 顾客, even though, yes, "traffic" to an online shop comes from customers.

Compare with English, in the same e-commerce context, where "traffic" alone is a fairly well understood and self-explanatory word.

线上: online

引: attract guests

流: customer flow

线上引流: attract customer online with various strategy to increase customer flow