Questions:
I have purchased $200,000 worth of shares of Apple from a broker by posting only a 40% margin. The broker requires a maintenance margin of 30%. They have told me that I need to ensure my deposit remains above the margin requirement or else the broker will margin call for more deposit. What does this mean?

At what value of my stock should I expect to receive the call for increasing the deposit?

Reference answers:
Maintenance margin, also known as minimum maintenance or variation margin, is the minimum amount of equity an investor must keep in a margin account to avoid a margin call.

Since you have borrowed the funds from the broker to invest in the Apple stock, they have a risk of losing this money. The maintenance margin will protect brokerage firms from default on this loan 

A margin call is a request from a broker for an investor to add more funds to their account or liquidate some of their holdings. If an investor doesn't meet the margin call, their position may be automatically liquidated.

To calculate the value at which the margin call will be received, the following formula can be used

Margin call price = Purchase Price (1 - initial margin / 1 - maintenance margin) 

= $200,000 (1 - 0.4 / 1-0.3)
= $171,428

So, if the value of your stock falls below this amount, you will be required to deposit funds to meet this minimum requirement.