The wand allows you to cast shield spells in addition to any you can cast yourself
The wand, as a magic item, typically allows you to cast a given spell a certain number of times (as defined by the individual wand), without needing to prepare or know the spell. For spells that require a spell attack roll or saving throw, with wands these would typically be fixed values.
Since you added the description of the wand, it tells you that you can cast shield from it as a Reaction as long as it has charges remaining (so at most 4 times a day, at the risk of losing it if you use the last charge).
How magic item charges work is explained on page 141 of the DMG:
Some magic items have charges that must be expended to activate their properties.
If you instead cast the spell with your spellcasting ability, you both need to prepare it (and you only can prepare a limited number of spells, see page 114 PHB) and it will use up a spell slot, of which you also only can use a limited number per day (for shield usually a first level slot). These spells use your own spell attack modifier and saving throw DC, if applicable (also on page 114).
Spell slots are explained on page 201 of the PHB:
Regardless of how many spells a caster knows or prepares, he or she can cast only a limited number of spells before resting. (…) Thus, each spellcasting class’s description (except that of the warlock) includes a table showing how many spell slots of each spell level a character can use at each character level. For example, the 3rd-level wizard Umara has four 1st-level spell slots and two 2nd-level slots.
When a character casts a spell, he or she expends
a slot of that spell’s level or higher, effectively “filling”
a slot with the spell.
Your cantrips are an exception to this. These are spells that require no spell slots, and can be cast as often as you like in a day. If they are a spell attack, this is the analogue to attacks with weapons by other classes and ensures the spellcaster has something useful to contribute, even after their limited spell slots run out. This is also on page 201 PHB:
A cantrip is a spell that can be cast at will, without using a spell slot and without being prepared in advance. Repeated practice has fixed the spell in the caster’s mind and infused the caster with the magic needed to produce the effect over and over. A cantrip’s spell level is 0.
Note that wand of shield is not one of the standard magic items from the DMG. If it is a homebrew item, it may have some unusual properties compared to the wands from the core rules, and my not be ideal to learn the rules.
For example, normally wands require your action (of which you have one per combat round, to Attack, Cast a Spell or do other things such as activating a magic item like a wand, see Actions in Combat, PHB p. 192). But shield is s spell that is usually cast with a Reaction, in response to being hit, and so this wand is activated as a Reaction, which is unusual.
Reactions are explained in page 190 PHB:
Certain special abilities, spells, and situations allow
you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else’s. The opportunity attack, described later in this chapter, is the most common type of reaction.
When you take a reaction, you can’t take another one until the start of your next turn. If the reaction interrupts another creature’s turn, that creature can continue its turn right after the reaction.