The  Hebrew word - marit ayin מַרְאִית עַיִן means 'appearance to the eye'. Certain actions, which might seem in reality to be fully permissible, are themselves not allowed in order to prevent onlookers from arriving at a false conclusion. This means there is a need to be aware of others around you.

In the book of Romans, Paul is discussing this very thing.

Romans 14:13 (TS2009)  Therefore let us not judge one another any longer, but rather judge this, not to put an obstacle or a stumbling-block in our brother’s way. 
14  I know and am persuaded in the Master יהושע that none at all is common of itself. But to him who regards whatever to be common, to him it is common. 
15  And if your brother is grieved because of your food, you are no longer walking in love. Do not by your food ruin the one for whom Messiah died. 
16  Do not then allow your good to be spoken of as evil. 
17  For the reign of Elohim is not eating and drinking, but righteousness and peace and joy in the Set-apart Spirit. 
18  For he who is serving Messiah in these matters is well-pleasing to Elohim and approved by men. 
19  So, then, let us pursue the matters of peace and the matters for building up one another. 
20  Do not destroy the work of Elohim for the sake of food. All indeed are clean, but evil to that man who eats so as to cause stumbling. 
21  It is good not to eat meat or drink wine, nor to do whatever by which your brother stumbles. 
22  Do you have belief? Have it to yourself before Elohim. Blessed is he who does not condemn himself in what he approves. 
23  But he who doubts, if he eats, is condemned, because it is not of belief, and all that is not of belief is sin.