Es que

Es que is useful construction for giving explanations or making excuses.

• ¿Ya no vienes? — Es que mi mamá necesita que le ayude con una cositas. I can't go because my mom needs me to help her.
• Tú y yo vamos a cambiar la cena para otra ocasión. Es que mi marido tiene un compromiso .... You and I will have to reschedule because my husband has plans. (From a telenovela)
• Es que tengo sueño. I can't. I'm sleepy.

Lo que pasa es que

There's also a longer version, lo que pasa es que. Example: Lo que pasa es que primero tengo que pasar por mi sobrina y luego a recoger unas cositas en el centro y de ahí ... así que hoy no voy a poder. The thing is (explaining why I can't do the thing you right now that have just asked me for), first I have to pick up my niece, then ...

Here's a non-excuse example. Imagine a friend is expressing frustration at the slow progress of his career. You could say Es que el éxito no llega de la noche a la mañana. Remember that success doesn't happen overnight. Notice that you're explaining why he hasn't had success yet. In other words, you're giving a reason.

Pretexto

Speaking of excuses, the word excusa exists, but don't overlook pretexto.

• ¡Nada de pretextos, eh! I read this on a post-it note. The meaning was: I won't accept any excuses, you must come to my party.
• ¡Puros pretextos!. Excuses, excuses!
• Más vale que tengas un buen pretexto. You'd better have a good explanation for this or else.
• No es la primera vez que me dejas plantada con cualquier pretexto.This isn't the first time you've stood me up with the flimsiest of excuses. (telenovela dialogue)

Don't be misled by the formality of the English word pretext. Pretexto in Spanish is perfectly useable even in informal situations.