What means the padlock ?
(I don't ask the function - only the sign)
(http://s24.postimg.org/xbs91yov5/Screenshot_2014_02_16_22_40.jpg) (http://postimg.org/image/xbs91yov5/)
Hold that button down for options and text but the padlock itself locks the zoom level.
To answer the question as asked though a padlock means nothing. It's a item used to secure things and is used in locus just because the icon makes sense I guess
I'm not sure I know on what you ask :)
Maybe little bit .. history. On place where is padlock, was before icon ... this http://www.katalogaplikaci.cz/Aplikace-1009-Locus-Free-pro-Android (first image). It was because there were only one function - zoom lock. Then come magnifier and later also "lock screen" and result was old icon without magnifier becuase we had experience that such icon was really confusing. This one is better - at least we do not receive any questions over email. This should mean "no problem" or "I won't touch this scary button". Both are good :)
mail sent
;D
mail? nothing, sorry ...